Please refer to FIG. 10, which is a sectional schematic view of an embodiment of an InGaAlP Schottky field effect transistor of conventional technology. The main structure of the InGaAlP Schottky field effect transistor 9 of conventional technology comprises: a compound semiconductor substrate 901, a buffer layer 902, a lower barrier layer 920, a first spacer layer 906, a channel layer 907, a second spacer layer 908, an upper barrier layer 930, a cap layer 912, a source electrode 916 (913), a drain electrode 917 (913), a gate recess 915 and a gate electrode 914. The compound semiconductor substrate 901 is made of GaAs. The buffer layer 902 is formed on the compound semiconductor substrate 901, wherein the buffer layer 902 is made of GaAs. The lower barrier layer 920 is formed on the buffer layer 902, wherein the lower barrier layer 920 is made of AlGaAs. The lower barrier layer 920 comprises a lower-barrier sub-layer 903, a lower carrier supply layer 904 and a lower-barrier spacer layer 905, wherein the lower-barrier sub-layer 903 is formed on the buffer layer 902; the lower carrier supply layer 904 is formed on the lower-barrier sub-layer 903; the lower-barrier spacer layer 905 is formed on the lower carrier supply layer 904. The first spacer layer 906 is formed on the lower-barrier spacer layer 905 of the lower barrier layer 920, wherein the first spacer layer 906 is made of GaAs. The channel layer 907 is formed on the first spacer layer 906, wherein the channel layer 907 is made of InGaAs. The second spacer layer 908 is formed on the channel layer 907, wherein the second spacer layer 908 is made of GaAs. The upper barrier layer 930 is formed on the second spacer layer 908, wherein the upper barrier layer 930 is made of InGaAlP. The upper barrier layer 930 comprises an upper-barrier spacer layer 909, an upper carrier supply layer 910 and a Schottky barrier layer 911, wherein the upper-barrier spacer layer 909 is formed on the second spacer layer 908; the upper carrier supply layer 910 is formed on the upper-barrier spacer layer 909; the Schottky barrier layer 911 is formed on the upper carrier supply layer 910. The cap layer 912 is formed on the Schottky barrier layer 911 of the upper barrier layer 930, wherein the cap layer 912 is made of GaAs. An ohmic electrode layer 913 is formed on the cap layer 912, wherein the ohmic electrode layer 913 and the cap layer 912 form an ohmic contact. Patterning the ohmic electrode layer 913 and then etching the cap layer 912 to form the gate recess 915, wherein the etching is stopped at the Schottky barrier layer 911 such that a bottom of the gate recess 915 is defined by the Schottky barrier layer 911. In the left side of FIG. 10, the ohmic electrode layer 913 and the cap layer 912 form the ohmic contact to form the source electrode 916; while, in the right side of FIG. 10, the ohmic electrode layer 913 and the cap layer 912 form the ohmic contact to form the drain electrode 917. The gate electrode 914 is formed on the Schottky barrier layer 911 within the gate recess 915. And the gate electrode 914 and the Schottky barrier layer 911 form a Schottky contact.
The material of the Schottky barrier layer 911 is based on InGaAlP instead of the more commonly used AlGaAs in conventional technology. The bandgap of InGaAlP is higher than the bandgap of AlGaAs, when comparing the field effect transistor 9 having the InGaAlP (higher bandgap) Schottky barrier layer 911 and the field effect transistor having the AlGaAs (lower bandgap) Schottky barrier layer, the field effect transistor 9 having the InGaAlP (higher bandgap) Schottky barrier layer 911 has a lower leakage current and a higher breakdown voltage. Since the material of the Schottky barrier layer 911 is InGaAlP which has a higher bandgap such that the bankgap difference between the Schottky barrier layer 911 and the cap layer 912 will be too huge. Hence, it will result in a barrier to the electron current and will affects the performance of the field effect transistor 9, such as increasing the on-state resistance (Ron), reducing the high speed switching ability and decreasing the microwave amplification gain.
Accordingly, the present invention has developed a new design which may avoid the above mentioned drawbacks, may significantly enhance the performance of the devices and may take into account economic considerations. Therefore, the present invention then has been invented.